In a textile yarn processing machine, particularly a two-for-one twister, it is desirable and has been the practice in some machines to provide a monitoring mechanism of the type generally described above for monitoring or sensing the presence of a running yarn in the yarn processing station and for controlling certain aspects of the operation of the textile machine, as for example by stopping the machine or the particular yarn processing station in the event of a yarn breakage.
In textile yarn processing machines, such as two-for-one twisters, in which the running yarn forms a rotating balloon of yarn during such processing, the yarn sensor is commonly associated with the yarn guide eyelet conventionally provided for limiting the upper end of the rotating balloon of yarn during passage of the running yarn through such eyelet. As a result of centrifugal forces acting on the yarn by the rotating balloon of such yarn, a tension is created in the yarn in the zone of the rotating balloon of such yarn, which is also assisted by tension forces acting in the direction of travel of the yarn as a result of the movement of the yarn toward the take-up mechanism. These combined yarn tension forces are exerted on the yarn guide eyelet in the running direction of the yarn and may be utilized for yarn monitoring or sensing purposes to hold a yarn sensor in an operative setting when the yarn is running and for releasing the yarn sensor in the event of yarn breakage to actuate a switching mechanism controlling the operation of the machine.
As disclosed, for example, in U.S. Pat. No. 3,981,134, issued Sept. 21, 1976 and assigned to the assignee of the present invention, the yarn guide eyelet is mounted to the machine frame for pivotal upward or downward movement under the influence of the forces exerted on the yarn guide eyelet by the yarn running therethrough. During normal operation, the yarn tension holds the yarn guide eyelet upwardly in an operative setting. However, in the event of the absence of such tension, as upon a yarn breakage or the absence of a running yarn, the yarn guide eyelet swivels downwardly about a horizontal axis and engages an electrical microswitch which serves for triggering a switching mechanism and stopping the operation of certain mechanisms in the yarn processing station.
After the yarn has been threaded in and the yarn processing station restarted, the yarn tension forces will be restored and the yarn guide eyelet will be moved back upwardly to its operative setting, as a result of which the microswitch will be released and returned to a setting in which it is again ready for sensing a yarn breakage.
A particular problem in the operation of the yarn sensor mechanism frequently occurs at this time. Upon restarting of the yarn processing station, the yarn tension required for maintaining the yarn sensor in its operative setting is not built up instantaneously, but gradually as the moving elements, such as the rotating spindle and the take-up assembly, return to normal operating speed from a stopped condition. During this gradual build-up of the yarn tension, the yarn sensor may sometimes move for a short period back into its nonoperative or released setting, and as a result may trigger the stopping of the yarn processing station as though a yarn breakage had occurred.
It has therefore been necessary, in order to avoid this inadvertent and undesirable tripping of the switching mechanism, for the yarn sensor mechanism to be very delicately tuned or adjusted to the build-up of the yarn tension, and this has necessitated frequent maintenance in order to maintain the yarn sensor mechanism in proper adjustment. This problem is particularly troublesome where the yarn sensor mechanism employs an electrical switch, as in the above-mentioned United States patent or as in the arrangement shown in German Offenlegungsschrift 2,024,122.